A Generator of Synthesis Gas and Hydrogen
Based on a Radiation Burner

Diunggah oleh

Deskripsi:

The possibility of the conversion of methane into synthesis gas in rich methane–air mixtures
under conditions of stable surface combustion in a volumetric permeable matrix in a radiation field of locked
infrared irradiation is shown. It is suggested to use the combustion of hydrocarbons in a volumetric permeable
matrix as a simple and compact generator of synthesis gas and hydrogen.

Format Tersedia

A Generator of Synthesis Gas and Hydrogen
Based on a Radiation Burner

Diunggah oleh

Deskripsi:

The possibility of the conversion of methane into synthesis gas in rich methane–air mixtures
under conditions of stable surface combustion in a volumetric permeable matrix in a radiation field of locked
infrared irradiation is shown. It is suggested to use the combustion of hydrocarbons in a volumetric permeable
matrix as a simple and compact generator of synthesis gas and hydrogen.

Abstract—The possibility of the conversion of methane into synthesis gas in rich methane–air mixtures under conditions of stable surface combustion in a volumetric permeable matrix in a radiation field of locked infrared irradiation is shown. It is suggested to use the combustion of hydrocarbons in a volumetric permeable matrix as a simple and compact generator of synthesis gas and hydrogen.

DOI: 10.1134/S0040579510010033

INTRODUCTION

The widespread use of natural gas and hydrogen in energetics, transport, and as raw materials for the chemi cal industry is one of the primary contemporary techno logical tendencies. Currently, the worldwide hydrogen production capacity is valued at 100 million tons per year. An overwhelming share of hydrogen is produced in the processes of natural gas and coal conversion into synthesis gas, and also in reforming and other oil refining and pet rochemical processes [1]. However, as the importance of reforming declines and the role of hydrogenation pro cesses grows when switching to a new generation of pollu tion free high octane motor fuels with lower level of pol lution and a low content of aromatic compounds and sul fur, petrochemistry itself is changing, to an increasing extent, from a supplier of hydrogen to its consumer. At the same time, the quick and significant growth of hydrogen and synthesis gas consumption is predicted in other fields. For instance, the potential market for methanol only is estimated at 800 million tons per year, and this exceeds its present world production by 20 times [2]. Therefore, the growth of hydrogen production up to 400 million tons is expected by 2050, while it would require almost 700 mil lion tons of hydrogen to completely replace the hydrocar bon motor fuels (approximately 2200 million tons) con sumed in the world.

From an ecological point of view, it is attractive to use hydrogen in motor transport for the power supply of onboard fuel cell–based engines, whose efficiency comes to 55%, twice exceeding that of internal combustion engines [3]. However, two serious problem stand in the way of its practical realization: it is necessary to increase the capacity of onboard hydrogen storage systems up to 5–7 kg and to create an efficient infrastructure for its pro duction and distribution. At that, the production must be located maximally close to the points of consumption in

†Deceased.

order to avoid the transportation over significant distances and the storage of great volumes of hydrogen, as its daily losses come to approximately 5% even under good ther mal insulation. As the distance between fuel stations must not exceed 3.2 km in cities and 40 km on highways, besides thousands of fuel stations, it will be required to have hydrogen synthesis plants located near them or directly included in them.

The United States Department of Energy (DOE), actively advocating the use of hydrogen in transport, has made the reduction of its price to $2–3 per kilogram for the ultimate consumer a top priority. Every day an individ ual fuel station serves, on average, 150–170 automobiles; therefore, hundreds of effective autonomous hydrogen synthesis plants with a capacity at a level of 1500 kg/day will be required [4].

Because of their technical complexity and high energy consumption, the contemporary industrial hydrogen syn thesis methods, which are based on steam or steam–oxy gen natural gas conversion, will hardly be able to claim a significant role in such a scattered production of hydro gen. In hydrogen and synthesis gas–based technological processes, their share of the commercial product cost exceeds 60% of the total expenses, and an acceptable profitability level is reached only at huge productive capacities from 1 million tons per year and higher [5]. The other known methods, including electrolysis and the use of alternative energy sources, give a several times higher prime cost of produced hydrogen and have practically no industrial use [6].

Steam and steam–oxygen natural gas conversion can hardly be imagined as a hydrogen source even for station ary autonomous energy supply sources, which are pre dicted to play an important part in energetics in the near future. In addition, it is much harder to imagine the suc cessive advancement of hydrogen transport energetics on the basis of them. This situation has made engineers search intensively for new methods of hydrogen and syn

thesis gas production. In recent years, several processes based on natural gas conversion in different power sys tems, such as internal combustion engines, gas turbines, and jet propelled engines, were proposed [7]. In spite of some progress, the complexity of these technologies has permitted none of them to attain an industrial level.

In the present work, the possibility of the production of synthesis gas and hydrogen on the basis of natural gas con version during its surface combustion in a volumetric per meable matrix under conditions of locked infrared irradi ation is considered. Combustion on a standard plane per meable matrix surface is realized by means of radiation and convective heat transfer from the flame front to the incoming mixture through the chain flame–matrix–ini tial mixture. At that, the burning hot matrix surface is the source of intensive infrared irradiation. Such a combus tion scheme provides for a substantial reduction in the flame front temperature and, correspondingly, a decrease in the concentration of nitrogen oxides in the combustion products. However, at that, the combustion limits become narrower because of additional losses by radiation. New possibilities of surface combustion are opened up when deep volumetric matrixes are used [8–10]. In this case, the combustion proceeds in the matrix cavity under the conditions of partially or completely locked infrared irra diation, and this method permits, if not to exclude com pletely but at least reduce them by many times, irradiation losses and, thereby, the substantial expansion of the com bustion limits. The experiment confirms the expansion of the limits towards the regions of both lean and rich mix tures [11].

In a volumetric matrix made from a porous material (ceramics or metal), the flame front can be observed both over the irradiating surface inside the matrix cavity and under the surface in the matrix body. At that, the process parameters (mixture composition and combustion tem perature) can be adjusted within very large intervals with the retention of combustion stability, which is impossible under conventional conditions. Earlier, this approach was used for the development of effective gas burners with a

low specific fuel consumption and carbon oxide emission

[8–10].

The purpose of the present work is to demonstrate the possibility of achieving high yields of Н 2and CO during methane conversion inside a volumetric honeycomb matrix cavity under conditions of locked infrared irradia tion and to estimate the prospects of the application of a similar device as a simple autonomous source of hydrogen and synthesis gas.

EXPERIMENTAL TECHNIQUE

The experimental test bench scheme is shown in Fig. 1. In this work, we used network gas as a fuel and air as an oxi dizer. The fuel and oxidizer rates were adjusted according to the readings of rotameters and were more precisely measured by gas meters. After leaving the mixer, a homo geneous air–gas mixture of a specified composition was fed into the radiation burner with a deep volumetric matrix (Fig. 2).

The main part of the experiments was performed on a burner with a matrix whose internal cavity represented an 80 × 40 ×115 mm right angled parallelepiped confined by walls, a bottom, and a top of perforated 15 mm thick ceramic permeated by cylindrical channels 1.2 mm in diameter. The ratio of the total channel cross section area to the total surface area (porosity) was γ = S c/S = 0.25. We worked with a burner which was open or covered by a per forated ceramic top 5. The air flow rate through the burner was 31–36 l/min in most experiments. All of the experi ments were performed at atmospheric pressure. The tem peratures of the matrix’s working surface and the combus tion products inside the burner device cavity were mea sured by chromel–alumel thermocouples.

For analysis, the conversion products were directly taken from the burner cavity by syringes of 5 cm 3in vol ume through a stainless steel capillary tube. The analysis of the products was conducted on an LKhM 8 chromato graph. Hydrogen, CO, air components, and hydrocar bons were determined on a 3 m long column with NaX

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Synthesis gas

3

5

2

4

1

Hydrocarbon–oxidizer

mixture

Fig. 2. Scheme of a radiation burner with a deep volumet ric matrix: 1 is the external shell, 2 is the volume with a gas–air mixture, 3 is the gas outlet, 4 is the burner cavity side walls and bottom made from perforated ceramic, and 5 is the burner cavity top made from perforated ceramic.

T, °C

1200

1000

1

800

600

400

2

200

0

0.4

0.6

0.8

α

Fig. 3. Dependence of (1) the temperature inside the opened burner cavity behind the flame front and (2) the temperature of the internal matrix surface on the oxidizer excess coefficient α.

molecular sieves, and СО 2and ethylene were defined on a 2 m long column with Poparack Q. The temperature of

the columns was 50°С; argon was used as a carrier gas.

EXPERIMENTAL RESULTS

The main value defining the temperature and the character of the chemical conversion processes in a burner is the oxidizer excess coefficient α = [О 2] 0/2[СН 4] 0, showing the deviation of the mixture composition from the stoichiometric ratio. If in the case of power plants, in order to use chemical energy more completely and reduce harmful emissions from their incomplete combustion, as a rule, the burning of fuel at

values of α ≥ 1 (stoichiometric and lean mixtures) is sought, then for the production of chemical products, such as synthesis gas and hydrogen, it is necessary to con

duct the process at α < 1 (rich mixtures). The lowest values of α at which we succeeded in achieving stable burner operation in air and obtained well reproduced results under conditions of the present work for the given burner

device were 0.35–0.37.

The temperature measurement experiments have

showed that when the parameter α is varied from 0.35 to

1, the surface temperature of the matrix changes from 350

to 650°С with the maximum at α ≈ 1. At α = 0.4, it is close to 400°С (Fig. 3). The temperature of the combustion products inside the cavity exceeds that for the matrix working surface and grows with an increase in α in the region of α ≤ 1. Note that the flame front temperature excess of at least 200–300°С over the irradiating surface temperature is also typical for the mixture combustion above the plane matrix surface.

The adiabatic heating of the methane–air mixture combustion products is equal to ΔТ р≈ 870°С at P =

1 atm, α = 0.4, and a thermodynamic equilibrium com

position of the products. However, the analysis of the product compositions shows that the system does not

attain a thermodynamic equilibrium over the time of res

idence in the flame front (Fig. 4). The calculation for α = 0.4, taking into account the experimentally obtained final

600°С), and this is most likely caused by the recuperation of flue gas heat as a result of its radiative transfer into the

matrix and further by the incoming fresh gas heated in the

channels of the burning hot matrix. Inside the burner cav

ity covered by a perforated ceramic top, the stabilized

temperature of the matrix surface and the products behind the flame front is approximately 100°С higher

than that in the burner with an opened cavity due to the

higher degree of reaction heat recuperation.

In spite of the fact that chemical equilibrium is not attained in the flame front, the temperature of the com bustion products in a locked radiation field is almost the same over the whole burner device cavity.

Consequently, the key feature of the given process is the possibility of the stable conversion of hydrocarbons at

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C, vol. %

23

20

18

H 2

16

14

12

CO

CO 2

10

8

6

CH 4

4

2

C

2 H 4

0

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

α

Fig. 4. Dependence of the methane conversion product concentration С in the matrix cavity on the oxidizer excess coefficient α.

low values of α < 0.5 and the flame temperature (~600– 700°С). This opens up real prospects for its application as a generator of different chemical products and, first of all, of hydrogen and synthesis gas.

The experimental dependencies on the value α for the concentration of methane conversion products in the burner with an opened cavity are shown in Fig. 4. It can be seen that the Н 2and CO concentrations grow sharply with a decrease in α, and at α ≈ 0.4 they are close enough to the values that are adiabatically equilibrium ones under these conditions. Thus, the experimental Н 2concentra tion is 18–22% (at α = 0.4, its equilibrium concentration is ~25%) and the experimental CO concentration is 10– 11% (at α = 0.4, its equilibrium concentration is ~14%). Accordingly, the СО 2concentration decreases monoton ically and reaches 2% (its equilibrium concentration is

~2%).

It is necessary to note that after the completion of oxy gen conversion and the formation of the main products, their concentrations remain nearly constant throughout the burner’s internal cavity height up to its external edge (Fig. 5). At a temperature of ~600–700°С, the proceed ing of any gas phase processes with the participation of СО 2and Н 2О deep oxidation products, advancing the system towards thermodynamic equilibrium, during the period in which gases reside in the burner is unreal. Only a monotonic drop in the concentration of methane, which seems to be subjected to thermal pyrolysis under these anoxic conditions with the formation of heavier condensation products that cannot be registered by us, is observed. Undoubtedly, the kinetics of these processes must be studied in more detail. The kinetic equilibrium established in the system makes the withdrawal of the syn thesized target products with the retention of their attained high concentrations easier. At low values of α, the

conversion of oxygen is nearly complete, and its small reg istered concentration should be related to the disadvan tages of the analytical method connected with the substi tution of some hydrogen by air for the period between the withdrawal of the sample and its injection into a chro matograph.

The dependencies on α of the two main Н 2/СО and СО/СО 2ratios characterizing the composition of the syn thesized products are illustrated in Figs. 6 and 7. The Н 2/СО ratio grows monotonically and comes to 1.8–2.2 at α = 0.37 (the equilibrium H 2/CO ratio is 1.68 at α = 0.4 and 1.98 at α = 0.3). The СО/СО 2ratio also increases monotonically and reaches 5 at α = 0.37 (the equilibrium CO/CO 2ratio is 8.4 at α = 0.4). Hence, in spite of the fact that, on the whole, the thermodynamic equilibrium com position of the products is not attained, the main param eters characterizing the yield of CO and Н 2are close enough to their thermodynamic equilibrium values.

It should be noted that the concentrations of methane and ethylene, whose thermodynamic equilibrium con centrations are very close to zero under these conditions, grow with a decrease in α (Fig. 4). At α = 0.37, the resid ual СН 4concentration is 4.8–6.6%, corresponding to a reduction in its conversion from 100% at α = 1.0 to ≈77% at α = 0.37.

The Н 2formation selectivity estimated by the balance of hydrogen containing products is approximately equal to 40%, and this gives a hydrogen yield of ≈30% during methane conversion in the area of 80%. The CO forma tion selectivity evaluated by the balance of carbonic prod ucts is approximately equal to 50%, and this gives a carbon oxide yield of ≈35% relative to the initial carbon. These data obtained in the device, which is relatively simple in comparison with what is now applied in industrial tech

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25

20

H 2

15

CO

10

CH 4

5

C 2 H 4

CO 2

O 2

0

20

40

60

80

100

120

140

L, mm

Fig. 5. Dependence of the methane conversion product concentration С in the matrix cavity on the distance L to the burner bot tom at α = 0.37.

Fig. 7. Dependence of the СО/СО 2ratio on the oxidizer excess coefficient α.

nologies, allow one to look forward to good practical pros

pects for the process.

The ethylene formation selectivity, which is high in

relation to the initial carbon and is equal to nearly 20%,

gives an ethylene yield of approximately 15%. However, in

the burner whose cavity is covered by a metallic grid, the

ethylene concentration in the products drops nearly to

zero. This is most likely caused by the fact that the internal

burner cavity temperature exceeds that of the “ethylene

gap” (>1100°С), connected with the change in the mech anism of the interaction between methyl radicals and oxy

gen [12]. At that, traces of ethane, which was not regis

tered under other conditions, appear. However, in the burner cavity covered by a ceramic top, the ethylene yield

remains high and the concentrations of the remaining reaction products do not substantially differ from those obtained for the variant with the open cavity.

A series of experiments were performed in a cylindrical burner with an internal diameter of 64 mm, a cavity depth of 60 mm, and a matrix made from a corrugated metallic nonwoven material (metallic felt of Nichrome wire 40 μm

in diameter with a porosity of 0.9–0.95). At that, no

appreciable distinctions were revealed both in the flame temperature inside the burner cavity and in the concen

trations of the formed products in comparison with their

values for the above described burner consisting of right

angled ceramic honeycombs.

RESULTS AND DISCUSSION

Historically, the combustion (oxidation) of hydrocar

bons was mainly considered as an energy source, and

most attention was concentrated on the kinetics of the processes in stoichiometric or lean mixtures, allowing for the most effective transformation of chemical energy fuel into thermal energy. Only a relatively short time ago did the

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hydrocarbon oxidation processes come to be regarded as a source of chemical products [13]. In any case, the possibil ity of the synthesis of important products in the oxidation of hydrocarbons is connected with their incomplete (par tial) oxidation; i.e., with the conversion of rich mixtures, mainly proceeding under severely nonequilibrium condi tions, when the yield of the products is caused by the kinet ics of the process. The prospects for the creation of new technologies for the synthesis of different products (hydro gen, synthesis gas, oxygenates, olefins, and others) by the incomplete oxidation of hydrocarbons have stimulated interest in the technological aspects of rich hydrocarbon mixture oxidation (combustion) and in the kinetics of these processes (see, for instance, [6, 12–15]).

With respect to the synthesis of the most demanded conversion products (synthesis gas and hydrogen), the main technological problem is the organization of stable hydrocarbon conversion at a low coefficient α = 0.3–0.5, which is necessary for attaining a high yield of these prod ucts. This is not a trivial task, as a converted mixture is practically outside the ignition region under conventional conditions at these values of α. A number of effective methods allow for the expansion of the region of stable combustion. Among them are the stepwise combustion of the mixture, the preliminary heating of gases, the com bustion of gases under increased pressures, the flue gas heat recuperation by a fresh mixture, the catalytic activa tion or chemical promotion of the flammable mixture, the ignition of the mixture by flame or by combustion prod ucts from an independent source, and the activation of the mixture by different physical methods. All of these meth ods, to one extent or another, are applied for the organiza tion of stable oxidation process, and in most cases several methods are used simultaneously. Some of the most typi cal methods are briefly described below for comparison with the proposed one. Autothermal reforming, which is a combination of partial methane oxidation and catalytic steam methane reforming, is a widely used industrial technology for the production of synthesis gas and hydrogen on the basis of partial methane oxidation. In this process, a low total value of α is attained due to the stepwise partial com bustion of methane under conditions close to stoichio metric ones with the subsequent addition of methane and steam to the hot oxidation products. The further catalytic conversion of heated gases allows for the obtainment of a nearly equilibrium composition of products. The original variant of this process was devel oped by the Halder Topsoe Company [16], which remains one of its leading developers.

The conversion of methane–air mixtures preliminary heated to 450–600°С into synthesis gas in a high pressure bomb (P 0= 30 atm) at α ≥ 0.35 in a self ignition regime

and

at

lower

α

<

0.25 in a forced ignition regime is

described in work [15]. The yields of Н 2and CO did not

depend on the ignition method and were close to the ther modynamic equilibrium values. The deviation from the

equilibrium values did not exceed several percent, but grew with a decrease in α. The authors drew the conclu sion that it was not reasonable to work at very low values of α ≤ 0.3, as an appreciable soot formation was observed for this regime, and at α = 0.25 the residual methane concen tration was already 11%.

In the case of natural gas conversion in an ener gochemical internal combustion engine based on aggre gates, in spite of the increased pressure (mixture compres sion ratio of 37–40), the possibility of working at low val ues of α ≥ 0.35 is provided either by the preliminary heating of the mixture to 100°С or by its forced spark or precombustion chamber ignition. The methane conver sion is 82–86%. At α ≈ 0.42, the composition of the prod ucts is as follows (vol %): 16.6 Н 2, 11.2 CO, 2.5 CO 2, 3.2 CH 4, and the H 2/CO ratio is 1.5 [6, 15].

In the aggregates based on rocket technologies (liquid propellant rocket engines), at α = 0.35–0.45 the possibil ity of the methane–air mixture conversion is mainly reached owing to an increased pressure inside the com bustion chamber and the continuous work of an incendi ary device (separate small power liquid propellant rocket engine). An additional contribution is made by the heat ing of the preliminary oxidant owing to the partial recu peration of flue gas heat transferred through the combus tion chamber wall cooled from the outside by the oxidant flow and, evidently, by the high degree of reacting mixture turbulization. The methane conversion comes to 81– 85%. The outlet concentration of H 2is 52–58% and that of CO is 35–38% for work with a methane–oxygen mix ture at α = 0.35–0.45 [6, 15].

For the processes of partial hydrocarbon oxidation into synthesis gas, the initialization by the “volley” injec tion of radicals or a molecular promoter can hardly be effective, as it requires very high concentrations for an introduced promoter [17], evidently, due to the short chains in these reactions. A wide range of woks is devoted to the investigation of the possibility of synthesizing hydrogen by the conversion of hydrocarbon gases in the plasma of an electrical discharge (see [18]). For example, in [19] hydrogen was obtained in the plasma of an arc dis charge with the use of a granulated nickel catalyst located in the postplasma zone. The energy efficiency of hydro gen emission was 1.21 MJ/kg at a methane conversion of 90% and a H 2yield of 90%. However, as for the chemical initialization, it is necessary to continuously introduce a lot of energy from an external source [18] to maintain the oxidation process because of the short reaction chains, and this fact makes these processes uninteresting for industrial application at present.

The catalytic activation of combustion processes, including that for rich mixtures, with the purpose of their conversion into synthesis gas is intensively studied. From this point of view, the processes proceeding at short times of contact with a monolithic catalyst (grids, sponge metal lic blocks, blocks with transparent channels) and, conse

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quently, at high hourly space velocities of a flow, are of especial interest. At millisecond contact times on similar catalysts, the oxidation processes pass nearly adiabatically in an autothermal regime with a high volumetric capacity. The process begins as a catalytic reaction on the surface and further continues in the gas phase at the expense of radicals and the active products formed on the catalyst [20]. On the sponge Rh/α Al 2O monolith, the methane conversion exceeds 90% at a Н 2and CO formation selec tivity that is also above 90% [21].

The intrareactor combustion product heat recupera tion is used in the filtration combustion processes to increase the temperature of the initial reagents and to extend the ignition limits. The possibility for the thermo chemical methane conversion into synthesis gas in the recuperative self sustaining regime in reactors of different types has been demonstrated. The values of α = 0.29– 0.33 are the optimal ones for the attainment of the maxi mal synthesis gas yield [22].

In works [8–11], the regularities of the gas–air mix ture combustion in volumetric matrixes under the condi tions of intensive radiative–convective heat exchange were investigated and the constructions of newly devel oped burner devices were described. The development of the results of these investigations in the given work has demonstrated the possibility of extending the combustion limits into the region of rich methane–air mixtures in the surface combustion regime in a volumetric permeable matrix under conditions of locked infrared irradiation. The nearly complete locking of irradiation inside the vol ume of the matrix, fulfilled in the form of a deep geomet rically locked cavity, reduces the radiation losses in the combustion by many times, and the appearing radiative positive feedback provides for a higher temperature of the internal surface of the matrix and the appropriate heating of the gas mixture in its channels.

The internal recuperation of the heat of the reaction proceeds at the expense of the partial combustion heat transfer to the inlet gas through the chain of the flame front (combustion products)–permeable matrix–gas passing through matrix channels. Hence, the inlet gas is heated before the flame front owing to the heat released during the combustion of the gas, which was burnt earlier and left the system and has been partially accumulated by the reactor cavity (matrix) walls. Therefore, such a chem ical system is open and the thermodynamic calculation of the products cannot be applied to it in principle. The products leaving the system have a lower temperature in comparison with that of the adiabatic combustion prod ucts due to the intensive cooling of the gas by means of convective and radiative heat withdrawal to the matrix walls. The internal matrix surface temperature is caused by the equilibrium between the heat generation by the gas combustion; the radiation heat wastes into the environ ment; and the heat entrainment by the cold gas, which passes through the matrix, becomes heated, and with an appropriate temperature enters the combustion zone.

This temperature may be accepted as the initial one for the gas entering into the flame front, although it can actually be higher at the expense of radiation and the convective heating of the gas directly before the front. In the flame front itself, the chemical processes proceed so quickly that they could be considered to be adiabatic if it were not for the intense radiative–convective exchange with the matrix surface. The initial heating of the gas at the expense of the reac tion heat recuperation and the reduction of radiation losses by several times during locking irradiation in the geometrically locked matrix cavity provide for the expan sion of the ignition limits and make the combustion of very rich mixtures with α ≤ 0.4 possible. Methane com bustion itself proceeds over millisecond time periods in the very narrow 1 to 2 mm flame front zone near the matrix surface. The composition of the products is caused solely by the kinetics of the gas phase combustion and, undoubtedly, is not equilibrium. Strongly endothermic processes, such as the reactions of steam or carbon diox ide reforming that play a great role in the establishment of a thermodynamic equilibrium in catalytic systems, can not produce any appreciable effect on the composition of products at ~700°С in the gas phase over the time of their residence in the burner. The temperature constancy in the volume behind the flame front results not from the fact that the system attains thermodynamic equilibrium, but from the temperature profile smoothing under the action of intensive equilib rium radiation in the locked matrix cavity. However, even in the absence of oxygen, at these temperatures behind the flame front the pyrolysis and residual methane condensa tion processes that produce a great number of different compounds from heavier hydrocarbons to soot can pro ceed at appreciable rates. In this work, we have shown the possibility of the application of a volumetric matrix for the organization of a stable combustion for rich methane–air mixtures at α ~ 0.4 and a low (500–700°С) temperature of combustion products with the obtainment of hydrogen and carbon oxide concentrations close to the thermodynamic equi librium values. The exclusion of radiation losses allows for the realization of the conversion for those mixtures whose combustion is impossible under conventional conditions. To put this into perspective, it is possible to change the gas mixture composition and the radiation field along the geometric coordinates of the volumetric matrix, which leads to the control of the distributed surface combustion regime and the optimization of the composition and yield of the target products. The obtained results have shown the possibility of effective natural gas conversion into synthesis gas with the close to optimal ratio of H 2/CO ≈ 2. When atmospheric air is used as an oxidizer, at the given convertor inlet the hydrogen concentration reaches 22% and that of carbon oxide comes to 11%; these values are both close to the thermodynamic equilibrium ones for these conditions.

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The principal possibility of the conversion of methane into ethylene with quite a high yield (up to 15%) in such a burner device was demonstrated. The resemblance of the results obtained in burners with different configurations and matrixes made from such materials as ceramic and metal, which are very different in respect to their catalytic properties, confirms the homogeneous character of the conversion process that proceeds mainly in the gas phase above the matrix surface.

Owing to the specificity of the conditions realized in a burner device with a volumetric matrix, the kinetic mod eling of oxidation methane conversion is extremely com plicated in it, and adequate results are hardly expected. Nevertheless, the results of the simulation of methane conversion in the flame of a diffusion air burner [23] are quite comparable to those of our experiment. When per forming the simulation, we obtained a sharp concentra tion maxima for Н 2and CO of 21.4 and 14%, respectively, at α = 0.4. At lower α values, these concentrations dropped sharply. As the calculation was conducted for a certain fixed time of the process, this most likely reflects the reduction of the rate of the process with a decrease in the temperature and the respective drop in the methane conversion during the given fixed time period. Some indi cations of an analogous reduction can be discovered in the respective experimental curves in Fig. 4. It is interesting that at α ≤ 0.4, a sharp reduction in the internal reactor temperature with the simultaneous drop in the yield is also observed in the synthesis of other products, for example, technical carbon [24] by partial natural gas oxidation. Evidently, in this case the decrease in the methane conver sion also is the main reason. Generators based on rocket technologies are the clos est analogs of a synthesis gas generator based on the com bustion of hydrocarbons in a radiation burner with a volu metric matrix. However, as distinct from the volumetric matrix–based radiation burners for which the reduction of radiation losses and the reaction heat recuperation are the primary factors influencing the expansion of the igni tion limits, for the generators based on rocket technolo gies such factors evidently are the increased pressure and high flow turbulization degree. In spite of the formal resemblance to the processes of catalytic combustion on grids or honeycomb block cata lysts, a principal difference exists between them and the combustion of hydrocarbons in a radiation burner with a volumetric matrix. The main factor providing for the sta bility of the catalytic conversion process is radicals, which are formed on the catalytic surface heated to a tempera ture above 1000°С and leave it for the reaction volume [17]. At the same time, neither the reduction of radiation losses nor the reaction heat recuperation plays any impor tant role here. Certainly, for the mixture combustion in a radiation burner with a ceramic or metallic volumetric matrix, one must not rule out the certain contribution of catalytic reactions inside the channels of the heated matrix, but the relatively low temperature (~400°С) of the

latter is hardly able to produce any appreciable initiating effect, even if it leads to the small conversion of the mix ture before the flame front.

On the grounds of the obtained results that coincide with the conclusions of works on the other methods of methane conversion into synthesis gas, one can draw the conclusion that the reaction mixture composition that is optimal for the production of synthesis gas in the partial oxidation processes lies within the interval of α = 0.3–0.4 regardless of the conversion method. Although from the thermodynamic standpoint lower values of α seem to be more favorable, as they give higher equilibrium Н 2and CO concentrations, in most cases these values are practi cally unattainable. Besides the abundant soot formation at α < 0.3, the quick descent of the temperature with a decrease in α leads to a drop in the methane conversion and, consequently, in the yield of Н 2and CO.

The burner device suggested in the given work is sim pler than the above described known synthesis gas pro duction methods, in particular, catalytic reforming, and at the same time, it is able to stably provide similar values of methane conversion and hydrogen and carbon oxide yield. Its productivity defined by the fuel mixture flow rate can be changed within wide intervals. And with an allow ance for the possibility to widely vary the burner power and the module construction of burner devices, similar hydrogen and synthesis gas generators are able to cover the whole interval of interesting capacities from small capac ity and even from microcapacity units to large chemical plants. At that, taking into account the gas phase charac ter of the process, such a scaling will hardly create any principal difficulties and significant changes in the unit’s capital costs.

Besides simplicity, compactness, and a high produc tivity, among the advantages of a generator based on a radiation burner with a volumetric matrix, one should include the process of autothermicity, the possibility to convert hydrocarbon gases with different compositions and even heavier hydrocarbons, and also the absence of the dirtying of the working matrix surface in the case of possible soot formation. Note that the conversion can be realized in a radiation burner with a volumetric matrix also at a high pressure. This makes this method attractive as a source of synthesis gas for industrial processes that require a high pressure of synthesis gas.

The device can be supplemented with a catalytic con verter of carbon oxide into hydrogen in the reaction with the steam, and this will allow for an increase of the hydro gen yield by 1.5 times. Such hydrogen generators are espe cially prospective for autonomous small capacity sys tems, for example, for distributed energy supply sources based on electrochemical generators (fuel cells) and for hydrogen fuel stations. At present, it is the absence of compact autonomous sources that is one of the main fac tors restraining the progress of hydrogen energetics.

THEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING

Vol. 44

No. 1

2010

28

ARUTYUNOV et al.

CONCLUSIONS

The possibility of methane conversion in very rich hydrocarbon–air mixtures in a stable surface combustion regime in a volumetric permeable matrix under condi tions of locked infrared irradiation was shown. The possi bility of controlling the composition of the combustion products and realizing the oxidation conversion for a mix ture whose combustion is practically impossible under conventional conditions was demonstrated. The effective conversion of natural gas by atmospheric air into synthesis gas with the ratio of H 2/CO ≈ 2 and the concentrations of hydrogen and carbon oxide of up to 22 and 11%, respec tively, was performed.

From the standpoint of the technological prospects for the process, its further development at increased pressures with the use of enriched air or oxygen as an oxidizer is of great importance. However, for economic and technolog ical reasons, the application of cheap nitrogen containing synthesis gas, which is produced during the oxidation by air and has the parameters close to those obtained in the given work (see, for instance, [25]), has good prospects, especially in the field of small capacity technologies.

It is possible that the above discussed relatively simple devices based on the matrix combustion of hydrocarbons and not connected with the use of cumbersome techno logical apparatuses and energetic aggregates will be able to not only fundamentally simplify the conversion process and increase its effectiveness, but be used for the produc tion of small hydrogen volumes. The practical creation of simple and efficient hydrogen and synthesis gas sources based on burner devices with a volumetric matrix for the conversion of gaseous and liquid hydrocarbons can make the wide application of electrochemical hydrogen based current sources with a power of several kilowatts to several megawatts realistic for the reliable energy supply of remote regions and for the autonomous energy supply of housing and communal services. The use of compact radiation matrix burner–based sources for the production of hydrogen from network gas or liquid hydrocarbons directly at fuel stations will allow for the transfer of the problem of hydrogen motor transport onto a practical plane.

ACKNOWLEDGMENTS

The authors are grateful to S.N. Podoinitsyn for his participation in performing the experiments.

NOTATION α—oxidizer excess coefficient;

γ = S c/S —matrix material porosity, which is equal to the ratio of channel cross section area S cto the total sur face area S;